Various question-answering systems applying natural language processing technique have been developed. In such a system, when a question sentence of a natural language sentence is given to the system, an answer is returned as a natural language sentence. At present, with remarkable advancement in computer and network techniques, conditions for wide-spread use of such a question-answering system have been satisfied. We cannot find, however, any question-answering system that is thoroughly penetrated among general users.
There may be various reasons for such unpopularity. By way of example, in a conventional question-answering system, a user makes a question sentence related to matters of his/her interest, presents it to the question-answering system, and receives its answer. When a question is given to the question-answering system in this manner, however, an accurate answer is rarely returned, and it is often the case that the user must try time and again. Such repeated failures discourage users who were at first interested in the question-answering system, and eventually, they cease to use the question-answering system. Further, if a user has a question of some sort, it is often difficult to express it in a specific question. A question-answering system returns an answer only when a question is given in some specific form. Therefore, even if one has a question of some sort about something, he/she cannot get an accurate answer unless that question is expressed in a form of a clear question sentence. Thus, after all, it is difficult to efficiently use the question-answering system.
On the other hand, a technique of information search using so-called keywords has been known. According to this technique, when a keyword is given to the system, sentences highly related to the keyword are retrieved from a database and presented to the user. This method also has a drawback that information appropriate as an answer to the question cannot be obtained unless a good keyword is found. Further, the obtained result is not an answer pin-pointing the answer to the question and, hence, the user has to read a large volume of retrieved documents.
As a technique assisting such keyword search, a so-called keyword suggesting function has been known, which suggests, when a keyword is input, those complementary keywords frequently input with that keyword. The user can improve search accuracy by selecting a suggested keyword.
Even when the keyword suggesting function is used, however, there is still a problem that relation between keywords cannot be made clear. Unless the situation in which a plurality of keywords are used could be specified, search accuracy would remain mediocre at best. After all, the problem that the user must read a huge amount of documents to find an answer cannot be solved.